Masataka Hirose

The valence band alignment at ultrathin SiO2/Si interfaces

High resolution x-ray photoelectron spectroscopy has been used to determine the valence band alignment at ultrathin SiO2/Si interfaces. In the oxide thickness range 1.6–4.4 nm the constant band-offset values of 4.49 and 4.43 eV have been obtained for the dry SiO2/Si(100) and the wet SiO2/Si(100) interfaces, respectively. The valence band alignment of dry SiO2/Si(111) (4.36 eV) is slightly smaller than the case of the dry SiO2/Si(100) interface.

Low‐temperature crystallization of doped a‐Si:H alloys

Crystallization of phosphorus‐doped a‐Si:H has been initiated at a substrate temperature below 200 °C, under the deposition conditions of a low flow rate of silane and in the presence of an external magnetic field. Along with the crystallization, the doping efficiency of the resulting Si:H films has been remarkably improved. Room‐temperature conductivity as high as 27 Ω−1 cm−1 has been achieved at a doping ratio of NPH3/NSiH4=5.6×10−3 for a specimen deposited at 30 C. Optical emission spectroscopy during the plasma deposition has revealed that a weak emission intensity of the SiH band with respect to hydrogen lines and the absence of emission from the doubly excited states of hydrogen molecules are necessary conditions for the crystallization of doped Si:H films.

Wide Optical-Gap, Photoconductive a-SixN1-x:H

The electrical and optical properties of a-SixN1-x:H prepared by the glow discharge of a SiH4-NH3-H2 gas mixture have been systematically investigated as a function of the ammonia-to-silane molar fraction. The optical bandgap increases smoothly from 1.75 eV to 5.5 eV as the molar fraction increases. A normalized photoconductivity ηµτ larger than 10-6 cm2V-1 is easily obtained in the ammonia fraction range 3.3×10-3 to 0.60, and it has a maximum (ηµτ4.8 ×10-5 cm2V1) around a fraction of 0.1.

Resonant tunneling through a self-assembled Si quantum dot

Nanometer-scale Si quantum dots have been spontaneously fabricated on SiO2 by controlling the early stages of low-pressure chemical vapor deposition from pure silane. The tunneling current through Au/1 nm-SiO2/a single Si quantum dot/1 nm-SiO2/n+-Si(100) double-barrier structures has exhibited the clear current bump or negative conductance at 300 K with a peak current to valley ratio as high as 10.

Analytic model of direct tunnel current through ultrathin gate oxides

A theoretical model for tunnel leakage current through 1.65–3.90-nm-thick gate oxides in metal-oxide-semiconductor structures has been developed. The electron effective mass in the oxide layer and the Fermi energy in the n+ poly-Si gate are the only two fitting parameters. It is shown that the calculated tunnel current is well fitted to the measured one over the entire oxide thickness range when the nonparabolic E-k dispersion relationship for the oxide band gap is employed. The electron effective mass in the oxide layer tends to increase as the oxide thickness decreases to less than 2.80 nm presumably due to the existence of compressive stress in the oxide layer near the SiO2/Si(100) interface.

Luminescence and transport in a-Si:H/a-Si1−xNx:H quantum well structures

Abstract Multiple-quantum-well (MQW) heterostructures consisting of twenty layers of a-Si:H (30∼200 A thick) and a-Si1−xNx:H (30∼200 A thick, x ≅ 0.2) have been studied by photoluminescence and current transport. The luminescence from the MQW structure exhibits a single peak at 1.37 eV originating in the a-Si:H well layers, and there is no emission at ∼1.48 eV arising from the a-Si1−xNx:H barrier layers. This is because the photocarriers generated in the barrier layers flow into the a-Si:H wells. This confinement of photocarriers in the quantum well has been demonstrated by analysing the luminescence quenching in the electric field applied perpendicularly to the MQW heterojunctions. The current transport parallel to the quantum well under light illumination has revealed the remarkable increase of photo-conductance with decreasing the well layer width. This is tentatively interpreted in terms of a significant increase in the mobility and lifetime of quasi-two-dimensional electrons in the a-Si:H well layers.

Characterization of plasma-deposited microcrystalline silicon

Abstract Microcrystalline silicon composed of crystalline and amorphous phases has been prepared by the glow discharge of a SiH4 + H2 gas mixture. The volume fraction of the crystallites and dark conductivity are simultaneously increased either by increasing the r.f. power or by decreasing the silane concentration. As the proportional content of the microcrystallites increases, only the number of crystallites is increased without any appreciable accompanying change in the grain size. On the basis of the structural model of microcrystalline silicon, it is suggested that the current transport of well-crystallized film is dominated by conduction in the crystallites. A nucleation mechanism of microcrystallization is discussed in conjunction with in-situ optical emission spectroscopy of the silane plasma.

Silicon Thin-Film Formation by Direct Photochemical Decomposition of Disilane

Silicon thin-films have been deposited by the direct photolysis of disilane at a substrate temperature below 300°C. The growth rate depends on irradiation intensity of a low pressure mercury-lamp, and a typical rate of 15 A/min has been obtained under ~0.08 watts/cm2 illumination, regardless of substrate temperature. The deposited films are composed of an amorphous network containing bonded-hydrogen in the range 6–9 at.%. The bonding configurations of SiH groups varied from silicon dihydride to monohydride with increasing substrate temperature, and correspondingly the dark conductivity decreased from 10-7 to 10-11 Ω-1cm-1. A broad photoluminescence peak at 1.4 eV was observed for a specimen grown at 200°C.

Electronic properties of chemically deposited polycrystalline silicon

For the systematic understanding of polycrystalline silicon, the conduction mechanisms, optical absorption processes, and recombination kinetics have been investigated. Tailing states as well as deep levels in the energy gap are found in activation energies of photoconductivity, high densities of ESR centers, and optical absorption tails at photon energies below the energy gap. A new model of electronic density‐of‐states distribution including the influence of the grain boundary has been constructed for the consistent interpretation of the observed characteristics. A substantial difference between the present model and a Schottky barrier model is also discussed.

Energy distribution of trapping states in polycrystalline silicon

The electronic density of states in the forbidden gap of polycrystalline silicon has been determined from an analysis of capacitance and conductance of a Metal/SiO2 (∼60 A)/polycrystalline silicon(∼250 A)/Si(111) (MOSS) structure. In this structure the thickness of the polycrystalline silicon is comparable to its grain size. Net density of trapped charges in the polycrystalline silicon is enough to terminate the electric field penetrating from the oxide layer. Then, two‐terminal admittance of the MOSS structure is dominated by charging or discharging of the trapping states in a wide range of applied gate bias. The U‐shaped distribution of trapping state density has been found for thin polycrystalline silicon films.